1. Field of the Invention
The present invention relates to, in general, a high pressure metal vapor arc lamp. In particular, the invention relates to a metal halide arc lamp containing a fill including mercury and metal halide materials such as NaI, ScI.sub.3, etc.
2. Description of the Prior Art
In general, metal halide arc lamps have a quartz arc tube enclosing a pair of electrode shafts therein. Each of electrode shafts faces each other in each side inside a tube, connecting with an individual external lead through a metalfoil. The metalfoil is made of high-melt point metals such as molybdenum. The each end of tube is squeezed to form a flat surface. The arc tube is filled with a starting rare gas such as argon, mercury, and a metal halide material such as NaI, ScI.sub.3.
To enclose a pair of main electrodes consisting of the electrode shafts and the metalfoils in an arc tube, the squeezed parts are formed by using the following procedures. Each of the main electrodes is faced each other along the elongated axis of the arc tube in each end of the arc tube. The each end is softened by heating and the each opposite side is squeezed with a pair of pinchers.
By using this method, a gap is created around the electrode shaft because the diameter of electrode shaft is large. The gap is created lengthwise along the electrode shaft. The width of gap extends to the breadth direction of the squeezed part, or to the direction perpendicular to the elongated axis of the arc tube. This gap is required to absorb a difference in the thermal expansion coefficient between the metal of electrode and the squeezed glass.
In a metal halide lamp with the above-described constructions, the metal halide enclosed in the arc tube is entered in the gap. When the lamp is energized, the temperature of the electrode shaft rises and the halide evaporates quickly. Evaporated halide provides high internal pressures to the narrow gap. When the compression strength of squeezed glass part is lower than an internal pressure, a shelly crack is created in the squeezed part. For some arc tubes, this crack caused filler in the arc tube to be leaked, or the arc tube to be damaged. In addition, since the thermal expansion coefficient of the electrode shaft is different from that of the squeezed part of tube, when an arc lamp was turned on and off, a crack was created in the glass of the squeezed part.
The result of observation and testing conducted by the investors shows that there is a trend toward smaller gap of a squeezed part with increasing leak and defect in arc tubes. The other result shows that larger gap lowers the initial pressure strength of the squeezed part of arc tube.